This invention relates to a method for initiating an in-situ recovery process or for starting the operation of the process to recover energy raw materials from a subterranean formation by the introduction of oxygen into the formation.
Since the invention of the underground combustion method for petroleum recovery by F. A. Howard in 1923, a number of methods have been developed, the object of which is the production of heat within the reservoir, especially of sufficient heat, by means of partial combustion of oil residues in a petroleum reservoir to enable recovery of the remaining oil. The most important processes contributing to petroleum displacement are viscosity reduction by means of heat, distillation and cracking of the oil and of the higher boiling components, sweeping out of the oil with hot water and extraction of the oil by means of miscible products. Such a method is specified, for example, in U.S. Pat. No. 3,026,935. Specific modifications of this method require a high oxygen partial pressure in order to bring about miscibility of the carbon dioxide formed during combustion. A high oxygen partial pressure can generally only be obtained by enriching the combustion-supporting gas with oxygen. Oxygen is known to be a gas which reacts readily with almost all substances. The amount of combustion heat released for example in a reaction between oxygen and organic fuels is considerable. On average it amounts to 3000 kcal per kg oxygen.
One of the disadvantages of the use of oxygen is its hazardous nature that could lead to uncontrolled reactions or explosions. Because of the hazardous nature of pure oxygen in reacting with other materials much work has been done to reduce this danger. In addition to the question of reaction of oxygen with various materials the dynamics of compressible fluids is also an important factor in determining what hazard exists when a material is reacted with oxygen.
Great importance is accordingly attached to the structure of the spaces in which the oxygen is flowing. Should said spaces possess a large inner surface in relation to the volume then the danger of an explosion when a fuel and oxygen are reacted is greatly reduced. Consequently the reaction of oxygen with oil contained in the pores of the reservoir rocks poses relatively few problems. However, given certain geometric proportions of the spaces through which the oxygen flows, local temperature peaks can occur, which, although not in accordance with the laws of the dynamics of compressible fluids, cause ignition of the material (steel, plastic, wood etc.).
Finally it is known from experience in autogenous gas cutting that not only the nature of the material but also the composition of the gas used has an influence on the material's cutting quality. With an oxygen content of less than 95%, steel can still be ignited but combustion is not self-sustaining. These ratios apply to atmospheric pressure. However there exists no practical experience with regard to high pressures as found in deep petroleum reservoirs.
If one proceeds from the assumption that the operation of oxygen plants above ground can be considered relatively safe and that the reaction of the oxygen with the oil in the reservoir can be controlled, then it follows that the most dangerous point along the oxygen's flow-path is the borehole. The operating conditions in a petroleum borehole are such that when high percentage oxygen is introduced there is a great danger of an explosion in the borehole. Neither is the borehole equipment made from deflagration-proof material (copper, Inconel) nor is the condition of the equipment, due to contact with corrosive, erosive and organic agents, such that the danger is lessened.
It is therefore the objective of this invention to eliminate these risks or at least reduce them to an acceptable level within the framework of conventional equipment used in boreholes for the recovery of energy raw materials such as petroleum hydrocarbons.